Heretofore, off-gas produced from a blast furnace is passed first through a primary cleaning device of the dustcatching variety for coarse particle removal and is then routed to secondary gas scrubbing and cooling devices. Following the cleaning and cooling, the off-gas is suitable for combustion in the air-heating section of a blast furnace and is often employed as a supplemental fuel therefor. The characteristics of off-gas vary depending upon the particular design of the furnace, its size and the particular operation being carried out.
A medium-sized blast furnace producing about a million tons a year of output is considered representative, and it might typically discharge 270,000 standard cubic feet per minute (scfm) of off-gas having a temperature between about 300.degree. F. (149.degree. C.) and 450.degree. F. (232.degree. C.) at a pressure of about 30 psia. In such an installation, gas cleaning is often accomplished using a venturi scrubber/separator to which water is supplied at a rate of about 10 gallons per minute (gpm) for each 1,000 scfm of off-gas. Accordingly, about 2700 gallons of water are being pumped to the venturi scrubber each minute of operation. Particulate matter and other contaminants carried in the off-gas stream are transferred to the scrubber water, and the exiting water stream may have a temperature of between about 115.degree. F. (46.degree. C.) and 155.degree. F. (68.degree. C.) and contain between 1,000 and 3,000 milligrams per liter (mg/l) of suspended solids.
In addition to the suspended material, the exiting scrubber water may representatively contain ammonia at a level of about 15 to 80 mg/l, phenol at about 0.05 to 0.3 mg/l and cyanide at a level of about 0.2 to 30 mg/l.
After scrubbing, the gas is commonly passed through a partially packed tower where it is subjected to countercurrent water flow which further lowers its temperature to about 95.degree. F. (35.degree. C.). Water may be supplied to such gas coolers at a rate of about 20 gpm/1,000 scfm of off-gas, i.e. about 5400 gpm. The exiting water may have a temperature of about 105.degree. F. (41.degree. C.) to 125.degree. F. (52.degree. C.), and it is fairly clean--having relatively minor amounts of particulate matter and low concentrations of ammonia, phenol and cyanide. Optionally, the off-gas may be further treated in precipitators, producing a discharge stream of water containing minor amounts of particulates and other contaminants.
The dirty scrubber water has generally been piped to a settling device, such as a clarifier or thickener, which separates the solids from the water as sludge. The sludge in slurry form is then de-watered, and the solids may be reprocessed with the sinter for recycle to the blast furnace. The clarified scrubber water and the gas cooler water are usually cooled and recycled. It is also acceptable to cascade a portion of the effluent from the gas cooler directly to the venturi scrubbers.
As the United States has become more concerned with environmental considerations, recycling of the cooling and scrubbing water has been required in order to reduce pollution. Contaminants which are contained in the effluent include heavy metals, phenols, cyanides, ammonia and fluorides, which are transferred to the water from the off-gas and the particulate matter. Generally, the recycling system is evaporative in nature because evaporative-type cooling towers are used, and thus there is a tendency to concentrate dissolved materials. It is well known that calcium and magnesium ions, as well as sulfate ions, upon concentration, give rise to mineral scaling and that to maintain adequate water quality for a recirculation stream some purging is necessary--the amount of water purged being referred to as "blowdown". Discharge of some water or blowdown is considered to be an economic necessity. Makeup water is necessary to replace both the amount purged and the amount which evaporates, and it is necessary to take into consideration the characteristics of the makeup water and the blast furnace gas system in order to properly balance blowdown, evaporation and makeup so as to control mineral scaling.
The rates of makeup, blowdown and evaporation have been used to define a parameter known as "cycles of concentration" which refers to a multiplier that, when applied to a constituent concentration in the makeup, produces a product which equates to the expected concentration of that constituent in the recirculation flow (and also in the blowdown). The levels of calcium and magnesium ions in the recycle stream increase in accordance with the above ratio and also because of transfer of matter from the off-gas to the water.
Effluent standards in the United States which apply to blast furnace systems regulate blowdown predicated upon the amount of iron produced in the blast furnace. Accordingly, the allowable number of pounds of a specific contaminant which can be discharged to the environment is based upon the production rate of iron, i.e., pounds of contaminant per 1000 pounds of iron produced. More stringent standards have been proposed, and it does not appear that many of these standards can be achieved simply by additional recycling and further limiting of blowdown. Such an attempt to simply increase the cycles of concentration in the recycled system would yield a water quality that would result in severe mineral scaling. Accordingly, improved methods for the treatment of the water in an off-gas cleansing system are needed, particularly in order to meet the proposed more stringent standards.